Component placement systems are utilized to place and align components such as integrated circuit (IC) devices in combination with printed circuit boards. An important operating consideration for placement systems is the degree of accuracy in component placement, i.e., lead-to-pad alignment.
Advances in integrated circuit architecture and miniaturization has resulted in greater numbers of functions being encompassed on single chips, necessitating the use of chips of larger dimensions, increased lead density, and/or reduced lead pitch. The increasing use of fine pitch integrated circuit devices (FPD), which presently have lead pitches on the order of 0.025 inches, and the projected use of FPDs having lead pitches on the order of 0.008 inches, has increased placement accuracy requirements for component placement systems.
Once a component has been properly aligned on a circuit board, the component must be bonded, for example by reflow soldering, to the circuit board. An important consideration in the bonding operation is the maintenance of the proper lead-to-pad alignment. Bonding may be accomplished by any of several methods.
The circuit board may be transported off the placement system to a mass reflow machine for subsequent bonding. Alternatively, selective reflow techniques may be utilized to bond the component prior to transporting the circuit board off the placement system. Or, the component may be reflow soldered at the time of placement utilizing a bonding subsystem integrally mounted with the placement system.
The third alternative is usually the most advantageous. Lead-to-pad misalignment may occur when a component is released after being placed and aligned, and/or when the circuit board is subjected to further handling prior to bonding. Reflowing to bond the component to the circuit board while the component is maintained in alignment by the placement apparatus significantly reduces or eliminates lead-to-pad misalignment problems.
The desire to provide bonding while the component is being held in place by the placement apparatus has generated a need for a bonding subassembly that may be mounted in combination with the placement head of the placement system. This imposes a design constraint on the bonding subassembly wherein the bonding subassembly must be configured for concentric disposition and operation with the placement head of the placement system. Concentric disposition and operation of the bonding subassembly would permit unimpeded operation of the pickup tool and image acquisition subassembly during both placement and soldering operations, thus ensuring initial lead-to-pad alignment as well as the maintenance of proper lead-to-pad alignment during the bonding operation.
Successful and cost effective implementation of an integral bonding subassembly in combination with a component placement system is influenced by the component size, component range, lead type, lead pitch, lead count, circuit board type, circuit board component mix, and circuit board topography, and the nature of the apparatus utilized to provide the thermal energy for the soldering operation. Different methods of providing thermal reflow energy are not necessarily suited to all applications inasmuch as the different methods provide different performance characteristics.
Performance characteristics such as programmability of heating, temperature control, temperature uniformity, thermal separation, packaging restrictions, and cycle time and cost should be considered in determining the type of bonding subassembly for use in integral combination with the placement system. Programmability of heating includes the capability to automatically or manually vary the pattern of heating to the component as well as the ability to program and vary heating parameters such as time and temperature on a component by component and/or site by site basis. Temperature control is the degree to which the temperature of the heating element and/or board can be controlled or predicted as well as the uniformity of the heat being delivered to the board. Thermal separation is the degree to which heating can be limited to the area of interest such as the component leads, thereby avoiding heating in undesired areas such as the component package and/or adjacent solder joints.
Packaging restrictions relate to the effective range of the heating method such as the sizes of component packages the subassembly is capable of handling, the proximity of the heating elements to the heating sites and potential interference with components on the board. Cycle time relates to the time required to bring the site to reflow temperature and cycle cost includes initial equipment costs, maintenance costs, and tooling costs.
Several different means of generating thermal energy for bonding of components to circuit boards may be utilized in an integral bonding subassembly including hot gas heating, contact heating and radiation heating. Hot gas heaters are available in various configurations, but the configuration most conducive to an integral soldering subassembly is a focused hot gas apparatus.
The hot gas apparatus consists of a linear array of orifices with electrical heating elements in each orifice to heat gas forced through the orifices. The gas flow is highly directional and selective operation of the various orifices and associated heating elements may be utilized to generate required hot gas heating patterns. While a focused hot gas apparatus generally provides the capability for programmable heating and reasonable thermal separation and cycle times, such apparatus have inherent limitations.
A focused hot air gas apparatus is generally a more complex system that requires precise regulation of the gas flow through the operational orifices, is limited in temperature control and subject to packaging restrictions. In general, such an apparatus has a very limited effective range due to dispersion and cooling of the gas flow. Dispersion and cooling effects engender concomitant difficulties in predicting and/or regulating the temperature at the bonding site and ensuring a uniform temperature at the bonding site. The limited effective range necessitates mounting of the apparatus in close proximity to the bottom of the placement head which may interfere with the operation of the pickup tool and/or image acquisition system. In addition, the kinetic energy of the heated gas flow may negatively impact the bonding procedure by disrupting or splattering the reflow material and/or causing misalignment problems due to inadvertent movement of the leads.
Contact heaters involve the use of heated blade edges which contact the leads and board. The blades may be heated by electrical resistance (thermode) or hot gas (convector). Contact heaters in general provide fast cycle times and excellent thermal separation. The configuration of the blades, however, limits the use of contact heaters to a very narrow range of component sizes. Alternatively, a separate mechanical positioning subsystem is required to move the blades to ensure contact with all of the leads of different sized components which adversely affects cycle time. In addition, the blades may interfere with the image acquisition subsystem during the actual reflowing bonding procedure. The blades physically contacting the leads of the component may also cause the leads to shifted out of alignment with the pads.
Radiation heating systems utilize the thermal energy of electromagnetic radiation to facilitate lead-to-pad bonding. Prior art radiation heating systems generally were not efficient, were not easily automated and/or did not provide good thermal separation. One prior art radiation system utilizes a mask that exposes the bonding sites while covering the remainder of the IC package and the immediately surrounding circuit board. Such a system requires a customized mask for each different type of IC package, and in consequence, such a system has relatively long cycle times and was not amenable to automation. In addition, such a system did not efficiently utilize the generated IR radiation since much of the radiation was reflected from the mask. Mask systems also required some type of means for dissipating radiation reflected by the mask.
Another type of IR heating system utilized a lens to focus the radiation as a spot encompassing the component and its leader. Due to the linear orientation of the bonding sites of typical IC packages, such a lens system is not readily amenable to providing IR radiation for linearly arranged bonding sites. Instead, such a system generally irradiated the entire IC package and the surrounding circuit board. Such systems did not efficiently utilize the generated IR radiation and did not provide good thermal separation.